The document WO 2011/009833 describes a method of identification of a material by scattering of X-rays. According to this method, a collimated polychromatic beam (or polyenergetic, the two terms being synonymous) of X-rays is directed towards a sample of a material to be identified. A part of the incident X-rays are subjected to Compton scattering inside of the material, and the photons scattered through a predefined scattering angle θ are detected by a spectrometric sensor. The collimated incident beam and the acceptance cone of the sensor, which is narrow (a few degrees), define an elementary volume inside of the sample, from where all the photons detected originate. The source of the beam of X-rays and the spectrometric sensor are displaced perpendicularly to a surface of the sample while keeping their orientation constant; in this way, the depth of said elementary volume is varied with respect to said surface of the sample; a spectrum of scattered X-rays is acquired for each said depth. The knowledge of two spectra corresponding to different depths allows two parameters μ′ and β, functions of the energy of the scattered photons, to be directly calculated.
It has been shown that, for a given energy and for a large number of known materials, μ′ is approximately proportional to the density of the material. This allows the density ρ—unknown—of the material to be characterized, to be estimated, starting from the value of μ′ averaged over a predefined range of energy. Furthermore, the knowledge of said density ρ and of the parameter β allows the ratio Zeff/Anorm to be determined, where Zeff is the effective atomic number of a material (a quantity which characterizes its interaction with X-rays) and Anorm its normalized molar mass. Thus, the material may be represented by a point in a plane ρ/(Zeff/Anorm), whose coordinates are determined based on at least two spectra of scattered X-rays. This allows the identification of the material.